1. Field of the Invention
The present invention relates to a small and light semiconductor device that can be manufactured at low costs, and is highly resistant to destruction even if excessively large electric power is supplied.
2. Background Art
FIG. 22 is a circuit diagram showing a conventional single-stage amplifier. An amplifying transistor 11 is a field effect transistor (FET) used for high-frequency waves of 0.1 GHz to 110 GHz, for example, the 2.1 GHz band. Input signals from the exterior are inputted in the gate (input terminal) of the amplifying transistor 11. The output signals of the amplifying transistor 11 are outputted to an antenna (not shown) from the drain (output terminal) via a matching circuit 27. Generally in communications systems and radars, a plurality of amplifiers are connected to the antenna to output electric waves.
However, if the antenna is positioned in the vicinity of a metal surface, outputted electromagnetic waves may be reflected and fed back to the amplifying transistor 11. In such a case, if large electric power is fed back, the amplifying transistor 11 may be destructed. To prevent this, an isolator 100 is connected to the output side of the amplifying transistor 11 (for example, refer to Japanese Patent Laid-Open No. 4-31782).
FIG. 23 is a graph showing the results of calculations for the input-output characteristics of a conventional amplifier at 2.1 GHz. In FIG. 23, Pin denotes input power, Po denotes output power, Id denotes drain current, and Ig denotes gate current. The gate width of the amplifying transistor 11 is 1 mm, and the maximum current Imax is 400 mA. It is known from these results that an average current of 50 mA/mm flows in the gate when Pin exceeds 25 dBm. Since the amplifying transistor 11 is destructed if such a large current flows, input of Pin not less than 25 dBm must be prevented.
FIG. 24 is a circuit diagram showing a conventional terminating resistor. An end of the terminating resistor 53 is grounded in terms of high-frequency waves. In microwave-band equipment, the resistance value of the terminating resistor 53 is 50Ω, which is common as impedance.
FIG. 25 is a circuit diagram showing a conventional T-shaped attenuator. First and second resistors 54 and 55 are connected in series. An end of a third resistor 56 is connected to the connecting point of the first resistor 54 and the second resistor 55, and the other end is grounded in terms of high-frequency waves. By selecting the resistance values of the first to third resistors 54 to 56, a desired attenuation can be obtained.